1. Technical Field
This invention relates to an epoxy resin composition for encapsulating a semiconductor chip and a semiconductor device. In particular, this invention is suitably used in an area mounting type semiconductor device where a semiconductor chip is mounted on one side of a printed-wiring board or metal lead frame and substantially only the mounted side is encapsulated with a resin.
2. Related Art
Recent market trend to size reduction, weight saving and higher performance in electronic devices has led to more integrated semiconductors. As surface mounting of a semiconductor device has been accelerated, there has been newly developed an area mounting type semiconductor device, with which a semiconductor device with a conventional structure has been replaced.
Such size reduction and thinning in a semiconductor device has demanded much lower viscosity and much more strength in an epoxy resin composition for encapsulating a semiconductor chip. In addition, because of environmental concerns, an epoxy resin composition for encapsulating a semiconductor chip has been increasingly required to be more flame-resistant without a flame retardant such as bromine-containing compounds and antimony oxide. Due to the background, a recent epoxy resin composition tends to contain a resin with lower viscosity and larger amount of an inorganic filler.
As new trend, a lead-free solder with higher melting point than that conventionally used has been increasingly used for mounting a semiconductor device. When applying the solder, the mounting temperature must be higher than that in a conventional process by about 20° C., sometimes leading to significantly lower reliability of a semiconductor device after mounting in comparison with a common device. In the light of such situation, it has been increasingly required to improve reliability of a semiconductor device by improving properties of an epoxy resin composition. For meeting the requirement, there have been investigations of reducing resin viscosity and filling larger amount of an inorganic filler.
Typical examples of an area mounting type semiconductor device include BGA (ball grid array) and CSP (chip scale package) with a further compacted size, and further packages such as QFN and SON with the smaller mounting area than conventional QFP or SOP. These packages such as QFN and SON were developed to solve the demands of high pin count and high-speed. These demands approach the limit in an area mounting type semiconductor device represented by conventional QFP, SOP and so on.
BGA and CSP are encapsulated by molding an epoxy resin composition on only the side, where the semiconductor chip is mounting, of a hard circuit board such as a circuit board consists of BT resin/copper foil (a bismaleimide-triazine resin/glass fabric substrate) is represented, of a flexible printed circuit board such as a circuit board consists of a polyimide resin film/copper foil is represented. Furthermore, on opposite side of side of a substrate on which a semiconductor chip is mounted, the solder ball is formed in parallel in two dimensions to be mounted on circuit board by soldering.
As described above, the structure of BGA or CSP is a one-side encapsulated structure where only a side of a substrate on which a semiconductor chip is mounted is encapsulated with an epoxy resin composition (and a side of a substrate on which a solder ball is formed is not encapsulated). Therefore, such a semiconductor device tends to be warped immediately after molding, due to unconformity in thermal expansion and thermal shrinkage between the organic substrate or the metal substrate and the cured epoxy-resin composition, or curing shrinkage of the epoxy resin composition during molding curing.
Furthermore, warpage in a semiconductor device causes a joining point of solder balls to be not located horizontally. Therefore, a semiconductor device is lifted from a circuit board during packaging these semiconductor devices by the solder joint on a circuit board, leading to deterioration in reliability of electric connection.
On the contrary, QFN or SON has been produced with the same design as conventional QFP or SOP. There have been recently, however, produced packages by mounting the matrix of semiconductor chips on one side of a metal substrate (for example, a laminate of a copper-lead frame, nickel-palladium+gold-plated lead frame stacked with a polyimide film), encapsulating them with an epoxy resin composition for encapsulating in one batch and cutting the substrate into lattice with a given size to give individual packages (hereinafter, referred to as MAP-QFN and MAP-SON) (for example, see Japanese Patent Laid-open No. 2003-109983).
As in the case of BGA or CSP, the structure of MAP-QFN or MAP-SON is a one-side encapsulated structure where only a side of a substrate on which a semiconductor chip is mounted is encapsulated with an epoxy resin composition. Here, an encapsulated area of MAP-QFN or MAP-SON is larger than common package molding and only one side is encapsulated. Therefore, such a semiconductor device tends to be warped immediately after molding, due to unconformity in thermal expansion and thermal shrinkage between the metal substrate and the cured epoxy-resin composition or curing shrinkage of the epoxy resin composition during molding curing.
Warpage in a semiconductor device causes a semiconductor device to be lifted from a circuit board for mounting that, leading to deterioration in reliability of electric connection.
For reducing warpage in an area mounting type semiconductor device in which substantially one side of an organic substrate or a metal substrate is encapsulated with an epoxy resin composition, it is important to approximate a thermal expansion coefficient of a substrate with that of a cured epoxy resin composition and to reduce curing shrinkage in an epoxy resin composition during molding curing.
To achieve them, there has been already suggested the technique that a multifunctional epoxy resin and a multifunctional phenol resin are combined to increase Tg of an epoxy resin composition and the content of an inorganic filler is adjusted to match α1. Combination of a multifunctional epoxy resin with a multifunctional phenol resin may, however, reduce fluidity to cause problems such as unfilled voids.
When soldering is conducted by solder processing such as infrared reflow, vapor phase soldering and solder immersion, moisture present within a semiconductor device due to moisture absorption by a cured epoxy resin composition (molding) is rapidly vaporized at elevated temperature. A stress generated during the process may cause cracks in the semiconductor device or peeling in an interface between the surface mounting the semiconductor chip in the metal substrate and the cured epoxy resin composition. Therefore, it is needed to reduce a warpage in a semiconductor device by increasing the content of an inorganic filler. It is needed to reduce a stress by reducing hygroscopicity of a molding. Furthermore, improvement in heat resistance of a molding and improved adhesiveness between a cured material and a metal substrate are also needed.
There has been developed the technique for maintaining high fluidity of an epoxy resin composition used in a surface-mounted semiconductor device such as conventional QFP and SOP during molding. For example, there have been disclosed using a resin with low melt viscosity (for example, see Japanese Patent Laid-open No. 1995-130919) and surface treatment of an inorganic filler with a silane coupling agent for increasing the content of an inorganic filler (for example, see Japanese Patent Laid-open No. 1996-20673). Any of these techniques can meet only one of various required properties.
As described above, it is necessary to add an inorganic filler at high concentration for cured material property improvement such as reduced warpage and reduced stress in a molding formed from an epoxy resin composition for encapsulating a semiconductor chip. Furthermore, for improving filling properties of an epoxy resin composition for encapsulating a semiconductor chip, its fluidity must be improved. However, when filling an inorganic filler at high concentration, fluidity of an epoxy resin composition for encapsulating a semiconductor chip is reduced. Thus, there is trade-off relationship between fluidity of an epoxy resin composition for encapsulating a semiconductor chip and cured material properties of a molding.
There have been still needs for an epoxy resin composition for encapsulating a semiconductor chip excellent in both fluidity and cured material properties in a molding, as well as a semiconductor device produced using the composition.